Background: Little is known about trajectories of cognitive functioning as individuals with Williams syndrome (WS) move though adulthood. Method: The present study investigated cognitive, linguistic and adaptive functioning in adults with WS aged 19-55 years, using both cross-sectional and longitudinal approaches. Results: Data from the cross-sectional study (n = 92; mean age = 32 years) indicated that IQ was comparable across age groups (Full-Scale IQ mean = 56-57) with Verbal IQ being slightly higher than Performance IQ. Daily Living Skills (as measured by the Vineland Adaptive Behavior Scales) were significantly higher in older individuals. Language abilities showed no consistent age-related differences. On formal tests of language, comprehension scores were higher than expressive language scores for almost all individuals, although this pattern was not replicated on the Vineland. In the longitudinal study, a follow-up of 47 individuals (mean age = 37 years) first assessed 12 years previously, similar trajectories were found. IQ remained very stable (FSIQ = 61-62 at both time points); there were significant improvements on the Social and Daily Living domains of the Vineland and significant decreases in Maladaptive scores. There were no improvements in language over time. Conclusions: The data indicate that adults with WS (at least up to the age of 50 years) show no evidence of deterioration in cognitive skills. Adaptive abilities continue to develop although language shows relatively little improvement with time.